In a typical projection-microlithography system, the pattern to be projected onto the surface of an exposure-sensitive substrate is defined by a pattern master generally termed a “reticle,” also called a “mask.” In the microlithography system the reticle is mounted on a stage that is capable of undergoing highly accurate movements as required during the lithographic exposure. While mounted on the reticle stage, the reticle is illuminated by a radiation beam (e.g., a beam of deep-ultraviolet or vacuum-ultraviolet light). As the beam propagates downstream from the reticle, the beam carries an aerial image of the illuminated pattern. This downstream beam, called a “patterned beam” or “imaging beam,” passes through a projection-optical system that conditions and shapes the patterned beam as required to form an image of the pattern on the surface of an exposure-sensitive lithographic substrate (e.g., a resist-coated semiconductor wafer or plate). For exposure, the substrate also is mounted on a respective movable stage called a “substrate stage” or “wafer stage.”
For holding the reticle (usually horizontally) during the making of lithographic exposures, the reticle stage is equipped with a “reticle chuck” mounted to a moving surface of the reticle stage. The reticle chuck holds the reticle suitably for imaging, without damaging the delicate reticle. For example, some reticle chucks hold the reticle by application of vacuum “force.” Other reticle chucks hold the reticle by electrostatic or Lorentz-force attraction. In microlithography systems in which the radiation beam is transmitted through the reticle, the reticle usually is held around its periphery (or at least along two opposing sides of the reticle) to avoid blocking light propagating to and from the reticle.
Two important measures of performance of a microlithography system are overlay and image quality. Image quality encompasses any of various parameters such as image resolution, fidelity, sharpness, contrast, and the like. “Overlay” pertains to the accuracy and precision with which a current image is placed relative to a target location for the image. For example, proper overlay requires that the current image be in registration with a previously formed, underlying structure on the substrate.
The manner in which and accuracy with which a reticle is held by a reticle chuck impacts various parameters such as image overlay and image quality on the lithographic substrate. Holding a reticle around its periphery or along two opposing sides leaves middle regions of the reticle unsupported and hence susceptible to gravitational sag. Fortunately, some of these consequences are readily modeled for behavior predictions from which offsetting corrections can be made. For example, a gravitationally sagging central region of the reticle tends to have an ideal deformed shape that is at most second-order (parabolic) about the scanning axis (y-axis) of the reticle. This deformation is consistent and predictable, and can be offset by making appropriate adjustments and/or compensations elsewhere in the system (e.g., in the projection-optical system).
As a reticle is being held on the reticle chuck, it is important to prevent movements of the reticle relative to the chuck and movements of the chuck relative to the reticle stage. Preventing such movements is especially important whenever the reticle stage is accelerating or decelerating while holding a reticle. Desirably, the reticle chuck mounted to the reticle stage produces substantial localized frictional forces at regions of contact of the reticle with the surface of the chuck. One conventional way in which these frictional forces are produced is to hold the reticle to the reticle chuck by vacuum-suction. Whereas this technique can produce effective reticle-holding friction, the reticle may be difficult to remove from the reticle chuck, such as after vacuum-suction has been turned off (e.g., before replacing a reticle currently on the reticle stage with another reticle).
One approach for obtaining reticle-holding friction with chucks employing vacuum-suction is discussed in U.S. Pat. No. 6,480,260 to Donders et al., incorporated herein by reference. Two opposing-side (flanking) regions (relative to the y-direction, the scanning direction) of the reticle are held on the reticle stage using respective “compliant members.” The compliant members are strip-like and extend lengthwise along the respective side region of the reticle and along the respective side region of the reticle stage. One lateral side region of the compliant member is mounted to the respective side region of the reticle stage and the other lateral side region of the compliant member extends in a cantilever manner from the respective edge region of the reticle stage. Extending along the cantilevered side region of each compliant member and projecting upward are short walls that define a respective “vacuum space.” The corresponding under-side of the reticle rests on the top edges (“lands”) of the walls that collectively serve as respective “chuck surfaces,” on which the reticle is held by evacuating the vacuum spaces. The compliant members are somewhat resistant to gravitational sag but yield to a limited extent to conform to the shape of the reticle without deforming the reticle.
Another approach is discussed in Applicant's U.S. patent application Ser. No. 11/749,706, incorporated herein by reference, in which the reticle stage has a movable support surface to which a reticle chuck is mounted. More specifically, the reticle chuck is mounted on a distal region of a “membrane,” of which a proximal region is mounted to the support surface and a distal region extends from the support surface and at least partially supports the reticle chuck in a cantilever manner. The reticle chuck includes walls and lands defining at least one vacuum cavity and multiple upward-extending pins. The lands and pins contact and support respective regions of the reticle as the vacuum cavity is evacuated to a desired vacuum level. Evacuating the vacuum cavity causes the reticle to adhere to the lands and top surfaces of the pins. Meanwhile, the membrane yields to allow the lands and pins to conform to the shape of the reticle.
From the foregoing, the stability of the reticle on the reticle stage and whether the reticle can be accurately and precisely positioned each time for exposure generally require that the reticle be held securely to the chuck without causing or allowing an uncorrectable distortion of the reticle. But, there are times when it is desirable that the reticle be removable from the chuck without damaging the reticle or chuck. I.e., when using a vacuum chuck, there are situations (e.g., in which the vacuum suction has been turned off but the reticle and chuck are still very smooth and clean) in which the residual force holding the reticle to the chuck is too high to allow ready removal of the reticle from the chuck without significant risk of damaging the reticle chuck and/or the reticle itself.
Therefore, especially when using a vacuum chuck to hold a reticle, there is a need for, inter alia, devices and methods for facilitating separation of the reticle from the reticle chuck without damaging, fracturing, or otherwise degrading the performance of the reticle chuck and/or the reticle.